The present invention relates to a control device for use in a valve for controlling the flow of a fluid to obtain a desired flow, and in particular, for controlling the opening displacement of the valve.
Japanese laid-open patent publication Tokkaihei No. 2-72201 discloses an apparatus for controlling the valve opening displacement to correspond with an intended opening rate by detecting the actual opening displacement of the valve, the apparatus specifically being intended to bring under control the flow of a pressure fluid supplied to a hydraulic actuator for use with industrial and constructing machinery.
The flow controlled by a fluid control valve is dependent on the opening area of the valve (proportional to the opening displacement and the difference in pressure between the upstream and downstream. With the fluid control valve disclosed in said publication, when the intended flow is determined, the flow is calculated from the valve opening displacement and the upstream and downstream pressure, and the feed-back control of the valve opening is made so that the calculated flow may correspond to the intended flow.
However, in this case, any deflection arisen between the actual opening displacement and the opening rate detected by the sensor will cause an error between the calculated flow and the actually controlled flow.
If the valve and seat portion on which the valve is to be seated are subjected to gradual wear from good use for a long period of time, there may occur deviation between the actual opening displacement and the opening displacement detected by the sensor. In addition, if the temperature of a fluid around the sensor is unusually elevated, the sensor may fluctuate in its output property with the subsequent failure in corresponding to the amount of displacement.
In any of the cases, no correspondence is established between the controlled flow and the desired flow, thus resulting in no fair control permitted.